The acoustic origin of the smile

In many species, appeasement or submissive vocalizations have a high fundamental frequency in order that the sender may sound like an infant and thus elicit a parental helping response from its would-be antagonist Morton, Am. Nat. 111, (1977). High vocal tract resonances may also enhance the infantile character of the vocalization by seeming to originate from a shorter vocal tract. Higher resonances can be achieved by a trumpet-like flaring of the tract and/or by retracting the corners of the mouth. This latter gesture, I hypothesize, is the origin of the smile. Ritualization of the visual component of this acoustically motivated gesture accounts for the smile often being performed soundlessly by man and higher primates. The upshifting of the resonances due to mouth corner retraction and/or flaring was simulated using pulsed plasticine models of vocal tracts. Significantly, the ethological literature also documents a widely found cross-specific aggressive display that involves constriction and protrusion of the lips, which by lowering the resonances of the vocalization would enhance the impression that the sender was very large.